CPN1

CPN1 encodes the 50-kDa catalytic subunit of carboxypeptidase N, a regulatory B-type zinc metalloprotease that cleaves C-terminal arginine and lysine residues from plasma peptides—including the complement anaphylatoxins C3a and C5a, kinins, and kininogen-derived fragments—thereby inactivating or modulating their receptor-binding activity to control inflammation and vascular tone (PMID:14687935, PMID:27058005). Its crystal structure resolves an N-terminal catalytic domain joined to a C-terminal transthyretin β-sandwich domain; two surface loops restrict the active-site groove (explaining resistance to protein carboxypeptidase inhibitors), an S1' pocket favoring C-terminal Lys over Arg accounts for substrate preference, and a hydrophobic patch at the catalytic–transthyretin domain interface mediates assembly with the CPN2 regulatory subunit (PMID:17157876). In vivo, CPN1 enzymatic activity is required to inactivate C5a: knockout mice are hypersensitive to lethal, histamine-mediated anaphylactic shock that depends on C5a/C5aR rather than C3a/C3aR signaling (PMID:19414808). Loss-of-function CPN1 coding variants reduce plasma CPN activity and cause carboxypeptidase N deficiency and hereditary angioedema with normal C1 inhibitor, attributed to accumulation of bradykinin and anaphylatoxins (PMID:12560874, PMID:38445235). Beyond its plasma role, a mitochondrial and cytosolic pool of CPN1 is activated downstream of electron transport chain damage during cardiac ischemia-reperfusion, where it cleaves the complex I subunit NDUFS7 and the inner-membrane protein mitofilin, impairs oxidative phosphorylation and mitophagy, promotes mitochondrial permeability transition pore opening, and triggers apoptosis (PMID:31411917, PMID:35550199, PMID:42193929).

1. 1998 Medium

   Establishing the gene identity and genomic location of the carboxypeptidase N catalytic subunit was the prerequisite for any molecular dissection of the enzyme.

   Evidence PCR mapping on somatic cell hybrid DNA panel localizing CPN1 to chromosome 10

   PMID:9628828

   Open questions at the time

   • No functional or structural characterization at this stage
   • Subunit assembly with CPN2 not yet addressed
2. 2003 Medium

   Linking human carboxypeptidase N deficiency to CPN1 itself answered whether the enzymatic phenotype was attributable to the catalytic gene, identifying loss-of-function alleles including a mutation at a conserved active-site residue.

   Evidence Genomic sequencing of a biochemically confirmed deficient proband plus control allele-frequency analysis

   PMID:12560874

   Open questions at the time

   • Single proband; broader genotype-phenotype spectrum unresolved
   • Functional consequence of G178D not directly tested by enzymology
3. 2004 High

   Defining the enzyme's substrate spectrum established CPN as the plasma carboxypeptidase that inactivates anaphylatoxins, kinins, and vasoactive peptides by removing C-terminal basic residues.

   Evidence Synthesis of in vitro enzyme-activity data on C3a, C5a, and kinin cleavage

   PMID:14687935

   Open questions at the time

   • Relative physiological contribution of individual substrates not ranked
   • Does not establish which substrate dominates pathology in vivo
4. 2004 Medium

   Detecting CPN1 expression in early embryonic erythroid and hepatic cells before complement activation raised the possibility of a developmental role independent of its known anaphylatoxin-regulating function.

   Evidence In situ hybridization and protein detection across mouse embryonic stages

   PMID:15177117

   Open questions at the time

   • No developmental phenotype demonstrated from this expression data
   • Substrate or pathway in embryonic tissues not identified
5. 2006 High

   The crystal structure explained substrate selectivity and inhibitor resistance mechanistically and defined the surface that mediates CPN2 binding.

   Evidence 2.1 Å X-ray structure of the catalytic domain with bradykinin active-site modeling

   PMID:17157876

   Open questions at the time

   • Structure of the intact heterotetramer with CPN2 not determined
   • Catalytic mechanism inferred from modeling rather than substrate-bound complexes
6. 2009 High

   Genetic epistasis in knockout mice resolved which anaphylatoxin drives the lethal phenotype, placing CPN1 activity upstream of C5a/C5aR-mediated histamine release rather than the C3a axis.

   Evidence CPN1 knockout mice with C5aR-/- and C3aR-/- double knockouts, cobra venom factor and i.v. anaphylatoxin challenge, antihistamine rescue

   PMID:19414808

   Open questions at the time

   • Human relevance of mortality phenotype not directly tested
   • Contribution of kinin substrates to phenotype not dissected
7. 2016 Medium

   Identifying a specific kininogen-derived substrate and tying reduced CPN1 activity to a disease serum biomarker extended the enzyme's relevance beyond classical anaphylatoxin control.

   Evidence In vitro cleavage of KNG1K438-R457 by purified CPN1 with patient-serum validation and mass spectrometry

   PMID:27058005

   Open questions at the time

   • Causal role of CPN1 activity change in BRCA1-mutant cancer not established
   • Single lab; mechanism of activity reduction unknown
8. 2019 Medium

   Discovery that a mitochondrial CPN1 pool cleaves complex I subunit NDUFS7 during ischemia-reperfusion introduced an intracellular proteolytic function distinct from the plasma metalloprotease role.

   Evidence Rat heart ISC-REP model with MDL-28170 inhibition, mitochondrial fractionation, complex I activity assays, and NDUFS7/beclin-1 Western blotting

   PMID:31411917

   Open questions at the time

   • NDUFS7 cleavage shown functionally but not by direct in vitro reconstitution
   • Relationship between this calpain-like activity and the plasma carboxypeptidase function not reconciled
9. 2022 Medium

   Cardiomyocyte CPNS1 deletion and upstream ETC blockade together established that electron transport chain damage drives CPN1 activation and that CPN1/2 cleaves additional mitochondrial targets to impair OXPHOS and trigger apoptosis.

   Evidence Conditional CPNS1 knockout mice with proteomics, infarct sizing and MPTP assays; amobarbital ETC blockade with spectrin-cleavage and AIF-truncation activity reporters

   PMID:34997008 PMID:35550199

   Open questions at the time

   • Proteomic targets (paraplegin, sarcalumenin, complex III) identified as candidates without direct cleavage reconstitution
   • Single lab; human cardiac relevance not established
10. 2024 Medium

    Familial CPN1 variants causing hereditary angioedema with normal C1 inhibitor confirmed that reduced CPN activity drives bradykinin/anaphylatoxin accumulation and human disease.

    Evidence NGS/Sanger sequencing of four families, plasma CPN activity measurement, and segregation analysis

    PMID:38445235

    Open questions at the time

    • Functional impact of each variant on enzyme structure not biochemically resolved
    • Why partial (30-50%) activity loss produces symptoms not mechanistically explained
11. 2026 Medium

    Direct in vitro cleavage of mitofilin by CPN1, with in vivo correlation to MPTP opening, identified mitofilin as a direct substrate and positioned CPN1 proteolysis upstream of permeability transition in ischemic injury.

    Evidence In vitro mitofilin peptide cleavage with exogenous CPN1 plus DCD rat heart model with MDL-28170 inhibition, mitofilin Western blotting, MPTP and infarct assays

    PMID:42193929

    Open questions at the time

    • Single lab; human cardiac data absent
    • Molecular link between mitofilin loss and MPTP opening not fully resolved

• It remains unresolved how the secreted plasma carboxypeptidase activity and the intracellular ischemia-activated proteolytic activity attributed to CPN1 are mechanistically related, and whether they reflect the same protein, distinct pools, or a nomenclature overlap.
• No study directly connects the plasma metalloprotease and mitochondrial activities
• Structural basis of the intracellular activity is uncharacterized